1. Field of the Invention
The present invention relates to an improved method and apparatus which can increase the heat-resistance in hollow biaxially-oriented containers formed by blow molding. More particularly, the present invention concerns improvements by which stretch blow molded containers can be efficiently cooled after they have been heat set and before they are removed out of the mold means.
2. Description of the Related Art
There is known a biaxial stretch blow molding process in which a preform placed within a blow cavity mold is engaged at its bottom by a stretching rod. As the stretching rod is moved in a direction of the longitudinal axis of the preform, the preform is stretched along the longitudinal axis thereof. A blow core inserted into the neck portion of the preform jets pressurized fluid to stretch the preform laterally or in a direction of the transverse axis of the preform.
Such a hollow container may be used as a beverage bottle which is required to have such a heat-resistant property as preventing the bottle from being thermally shrunken or deformed since the content thereof is heated and charged into the bottle. In order to form a hollow container having such a heat-resistant property, it is known that it is useful to heat-set a stretch blow molded container, that is, to maintain such a container heated for a predetermined period of time to adjust the orientation of the polymer thereof.
There have been proposed three methods of molding hollow heat-resistant containers:
(1) A blow mold is first heated up to 130.degree. prior to stretch blow molding. After a container has been stretch blow molded, the temperature of the blow mold is cooled down to 100.degree. to prevent the molded container from being deformed on removal, as disclosed in Japanese Laid-Open Patent Application 77672/1979.
(2) After stretch blow molded, a hollow container is retained within a heated blow mold to remove the residual stress at the stretched portion of the container while maintaining somewhat internal pressure in the hollow container. Thereafter, the hollow container is transferred into a separate cooled mold so that the hollow container is cooled and solidified. For example, see Japanese Laid-Open Patent Application 33622/1987.
(3) A blow mold is previously heated up to a predetermined temperature. After a hollow container has been stretch blow molded in such a heated blow mold, a gas (e.g. liquefied carbon dioxide) is blown into the interior of the hollow container through a stretching rod at room temperature to quench the hollow container before it is removed out of the blow mold. For example, see Japanese Laid-Open Patent Application 93330/1984.
The first and second mentioned processes according to the prior art are disadvantageous in that the cooling time is relatively prolonged to reduce the efficiency thereof, because they require to cool the blow mold having its large heat capacity down to a predetermined temperature or to transfer a hollow molded container into a separate cooling mold. On the contrary, the third process of the prior art is advantageous in that the cooling time and thus the efficiency can be improved since the hollow molded container is quenched directly by the cooling gas, that is, liquefied carbon dioxide while being held within the heated blow mold.
In the Laid-Open Patent Application 93330/1984 disclosing the third mentioned process, there is shown a stretching rod which has a hollow portion extending from the base to the middle part thereof. A pipe for introducing a cooling medium is coaxially disposed within this hollow portion of the stretching rod. The cooling medium outlet formed in the pipe at one end is positioned near the neck portion of the hollow container when the axially forward movement of the stretching rod is terminated.
When the cooling medium outlet of the pipe is disposed near the neck portion of the hollow container to introduce the cooling medium into the hollow container, the entire hollow container having a given length will not be efficiently cooled by the cooling medium. It may be difficult to cool the hollow container uniformly since various portions of the container are differently cooled by the cooling medium.
Since the cooling medium is introduced into the hollow container near the neck portion thereof, only the limited parts of the hollow container are cooled by the blown cooling medium. This also causes the reduction of the efficiency on cooling. Such a tendency is increased if a hollow container having an increased rate of longitudinal stretch is to be cooled by the cooling medium.
Although the third mentioned process of the prior art utilizes the cooling gas at room temperature, it is preferred that cooling medium is used at lower temperatures to reduce the cooling time and to improve the efficiency on molding.